The present invention relates generally to laser radiation detectors and more particularly to laser-induced plasma generated electromotive force used for detector and monitoring of the temporal characteristics of pulsed laser output.
When a high-power laser is focused onto a target some portion of the target surface material is vaporized and ionized forming a plasma. Electrons appearing in the ionization process move much more rapidly than the simultaneously formed ions because of their smaller mass, and a certain fraction have sufficient energy to move away from the surface on which they originated leaving the ions behind. This effect can produce large voltages if the target is electrically insulated from any surrounding conducting surfaces. That is, the electrons striking the outlying walls can produce a voltage relative to the target material which is enveloped by ions which have not had time enough to leave its environs. A return circuit including a resistive element allows useful power to be generated as these electrons return to the target and recombine with the ions still there. It turns out that a significant number of electrons are traveling at superthermal velocities. That is, some of the electrons are moving much faster than would be predicted from the plasma temperature. There are many explanations for this phenomenon, none of which concerns us here. It will be seen, below, that these "hot" electrons give rise to the unique characteristics of our invention.
The device and method of the instant invention is a room temperature, fast, and robust laser radiation detector useful for the investigation of laser pulses. Electron emission from plasmas produced by focused, pulsed laser radiation gives rise to electrical signals which can be utilized for beam diagnostics or synchronization. The invention has a risetime, voltage output and response duration which can be adjusted by varying gas pressure and type. Under suitable conditions the detector can be made to have a faster risetime and longer accurate response duration than existing detectors using thermal electron characteristics. It can also be made more difficult to saturate. Pulses of 250 V amplitude into 50.OMEGA. can be generated without further amplification under conditions where the risetime is less than 0.2 ns using a 10.6 .mu.m laser source. At higher pressures, an approximately 1 V signal with even faster temporal characteristics reproduces a laser pulse train lasting 100 ns. The invention is therefore a good qualitative laser beam monitor as well as a detector. The absence of measurable delay time between the rise in detector signal and the incident laser pulse enable the device and method of the instant invention to be useful as a fast trigger source.
The generation of high voltage transients by focused CO.sub.2 laser radiation for energy conversion has been reported by W. T. Silfvast and L. H. Szeto in Appl. Phys. Letters 31, 726 (1977). The authors discuss therein the use of a device similar to that of the instant invention as a laser detector. However, this article teaches away from the method and device of the instant invention in several critical ways. First, the method proposed and executed by Silfvast and Szeto describes the evacuation of their detector cell to 10.sup.-2 torr or lower. We have found that by adding various gases to pressures as high as one atmosphere, the temporal resolution of the device can be significantly improved. The desired decrease in detector response time is accompanied by a reduction in the voltage generated for a particular laser energy, but even at atmospheric pressure, the signals are of the order of one volt. FIG. 3 of the Silfvast and Szeto article shows no fine structure on the detector output signal, while the laser pulse has significant structure. Our invention is capable of following such short-term laser fluctuations accurately because of the laser driven, non-thermal characteristics of the signals produced by the "hot" electron phenomenon. In fact, it should be noted that it provides subnanosecond response time and high voltage output even when evacuated, whereas Silfvast and Szeto report about 8 ns response for their detector. We have further discovered that by varying the gas pressure and its composition the duration of the detector response can be adjusted. It is seen from FIG. 3 of Silfvast and Szeto that their detector signal drops off in less than one-half the time of the laser pulse decay. The explanation tendered by the authors is that the space-charge effect prevents the high-energy electrons from reaching the detector walls at longer times. Since the instant invention relies on "hot" electron emissions, i.e., those electrons which have sufficient energy to pass through the gas present to the conducting walls, the space-charge effect becomes less important and the response duration increases. At still longer times, our invention can be made to cut off additional laser signals since the gas becomes conducting and effectively shorts the device. The detector is there acting as a plasma switch and terminates the 10.6 .mu.m radiation reaching the detector cathode. Control of detector response time and duration coupled with an approximately factor of 100 improvement in fastest response time, then, are the critical features which distinguish the method and device of the instant invention from existing technology. Finally, the dimensions of our device are such that the saturation effect graphically described by FIG. 5 of Silfvast and Szeto is not observed at the maximum intensity attainable by us during our experiments; namely, about 10.sup.13 W/cm.sup.2. In their FIG. 5, it is seen that saturation has already set in at this intensity level. That is, the change in detector signal per change in laser intensity becomes small. This advantage further distinguishes the device of the instant invention from that of Silfvast and Szeto.